…plus shoes, floors, roofing…
Typical products manufactured using recycled rubber fillers include insulation panels and rubber mats, flooring for sports halls, shoe soles, carpet backing materials...
Recycled rubber has to meet tough requirements to be suitable for the manufacture of high quality products. Industry requires powders with grain sizes of 100-600µm and with a very large specific surface area. Because of the chemical cross-linking produced by vulcanisation, recycled rubber can never have the same properties as ‘fresh’ raw materials, and is therefore effectively downgraded. In practice, this means that recycled rubber can only be used as a filler in new mixtures. So rubber is normally recycled by grinding to produce fine powders with a large specific surface area. Until now, that surface area could only be produced using ‘warm’ grinding processes whose yield of particles with grain sizes between 400 and 600µm was too low.
Since 1995, Mülsener Recycling of Mülsen St Jacob, Germany, has been operating a used tyre granulating plant. Every year, approximately 10,000 tons of used truck tyres and other industrial rubber waste are converted to powders and granulates of between 0.4 and 7mm. In 1996, the decision was taken to produce rubber powder for the manufacture of higher value products and a fine grinding stage was added. To produce grain sizes of 200-400µm cost-efficiently, this had to be a cryo-grinder.
A Jäckering Ultra-Rotor VI model universal mill was fitted with Cryogen equipment from Messer. The granulate is embrittled by cooling to -150°C with liquid nitrogen, and is then ground to a fine powder using impact mills fitted with a recirculation system for cold nitrogen gas.
From the first trials a marked increase in the throughput and the yield of granulates with a grain size of below 500µm was observed: cryogrinding produces a readily pourable material with a high proportion of fine grain granulate. Yet, because of the relatively smooth fracture surfaces of the particles, they have a low specific surface area. The high mechanical stress in ambient temperature grinding without cooling produces a very uneven surface, but the proportion of fine granulates and the throughput are both low.
To exploit the strengths of both grinding technologies, the cooling and grinding stages were decoupled. A high yield of fine grain granulate was produced by embrittling the rubber granulate in a screw cooler before passing it through a specially adapted grinding mill. The output from the mill has a temperature of 15°C, a high proportion of fine grain granulate and a large specific surface area.